1. Field of the Invention
The present invention relates to a method and an apparatus for the formation of a functional deposited film on a substrate by means of plasma chemical vapor deposition (hereinafter referred to simply as "plasma CVD"). More particularly, the present invention relates to a method for continuously forming a functional crystalline or non-single crystalline deposited film on a substrate by plasma CVD, the deposited film being usable as a semiconductor element for semiconductor devices, electrophotographic photosensitive devices (or electrophotographic light receiving members), photovoltaic devices, image input line sensors, image pickup devices, and thin-film transistors. The present invention also relates to an apparatus suitable for practicing said method.
2. Related Background Art
Heretofore, as the constituent element members of semiconductor devices, electrophotographic photosensitive devices (or electrophotographic light receiving members), image input line sensors, image pickup devices, or other electronic devices including optical devices, there have been proposed a number of non-single crystalline semiconductor deposited films. For example, an amorphous deposited film composed an amorphous silicon material compensated with hydrogen atoms (H) or/and halogen atoms (X) such as fluorine atoms or chlorine atoms [hereinafter referred to as "a-Si(H,X)"] and a number of crystalline semiconductor deposited films such as so-called diamond deposited thin films and polycrystalline silicon deposited thin films have been suggested. Some of these films have been put in practical use.
It is known that such semiconductor deposited films can be formed by means of a plasma CVD process in which a deposited film is formed on a substrate of glass, quartz, heat-resistant synthetic resin, stainless steel, or aluminum by decomposing a film-forming raw material gas with a glow discharge using a direct current (DC) energy, a high frequency energy or a microwave energy.
There have been proposed various film-forming apparatus suitable for practicing such plasma CVD process.
FIG. 1 is a schematic diagram illustrating a film-forming apparatus by a high-frequency plasma CVD process (this apparatus will be hereinafter referred to as high-frequency plasma CVD apparatus) as an example of such film-forming apparatus.
The high-frequency plasma CVD apparatus shown in FIG. 1 comprises a deposition system 2100 comprising a reaction chamber 2111 (or a deposition chamber) provided with means for evacuating the inside of the reaction chamber, and a raw material gas supply system 2200 for supplying raw material gas into the reaction chamber 2111.
The reaction chamber 2111 of the deposition system 2100 has a substantially enclosed reaction space. Reference numeral 2112 indicates a substrate (for instance, a cylindrical substrate) placed on a rotatable cylindrical substrate holder 2113 having an electric heater 2113' therein which is arranged in the reaction space of the reaction chamber 2111. In the reaction space of the reaction chamber 2111, there are installed a plurality of gas feed pipes 2114 each being provided with a plurality of openings (not shown) for feeding a raw material gas. The gas feed pipes 2114 are connected to a gas supply pipe 2116 provided with a sub-valve 2260, extending from the gas supply system 2200. The reaction chamber 2111 is provided with an exhaust pipe 2117 which is connected through a main valve 2118 to a vacuuming apparatus such as a vacuum pump (not shown). The exhaust pipe 2117 is provided with a leak valve 2117'. Reference numeral 2119 indicates a vacuum gage connected to the exhaust pipe 2117. Reference numeral 2115 indicates a high frequency matching box extending from a high frequency power source (not shown), being electrically coupled to the reaction chamber 2111.
The raw material gas supply system 2200 comprises gas reservoirs 2221 to 2226 respectively containing a given raw material gas, for instance, SiH.sub.4 gas in the gas reservoir 2221, GeH.sub.4 gas in the gas reservoir 2222, H.sub.2 gas in the gas reservoir 2223, CH.sub.4 gas in the gas reservoir 2224, B.sub.2 H.sub.6 gas diluted with H.sub.2 gas (hereinafter referred to as B.sub.2 H.sub.6 /H2 gas) in the gas reservoir 2225, and PH.sub.3 gas diluted with H.sub.2 gas (hereinafter referred to as PH.sub.3 /H.sub.2 gas) in the gas reservoir 2226; valves 2231 to 2236 for the gas reservoirs 2221 to 2226; inlet valves 2241 to 2246; exit valves 2251 to 2256; and mass flow controllers 2111 to 2216. Each of reference numerals 2261 to 2266 indicates a pressure controller. Each of the gas reservoirs 2221 to 2226 is communicates with the gas feed pipes 2114 in the reaction chamber 2111 through the gas supply pipe 2116 provided with the sub-valve 2260.
Film formation using the plasma CVD apparatus shown in FIG. 1 is conducted, for example, in the following manner.
A cylindrical substrate 2112 on which a film is to be deposited is positioned on the substrate holder 2113 in the reaction chamber 2111, followed by evacuating the inside of the reaction chamber 2111 to a desired vacuum by means of a vacuum pump (not shown). The cylindrical substrate 2112 is heated to a desired temperature (for example, a temperature in the range of 20 to 450.degree. C.) by actuating the heater 2113' and it is maintained at this temperature.
Prior to the entrance of film-forming raw material gases into the reaction space of the reaction chamber 2111, it is confirmed that valves 2231-2236 of the gas reservoirs 2221-2226 and the leak valve 2117' of the reaction chamber 2111 are closed and that the inlet valves 2241-2246, the exit valves 2251-2256 and the sub-valve 2260 are opened. Then, the main valve 2118 is opened and the vacuum pump (not shown) is actuated to evacuate the reaction space of the deposition chamber 2111 and the gas pipe ways. Upon observing that the reading on the vacuum gage 2119 became about 5.times.10.sup.-6 Torr, the sub-valve 2260 and the exit valves 2251-2256 are closed. Thereafter, the valves 2231-2236 are opened to introduce the respective raw material gases from the gas reservoirs 2221-2226, and the pressures for the respective raw material gas are adjusted to 2 Kg/cm.sup.2 by the pressure controllers 2261-2266. Then, the inlet valves 2241-2246 are gradually opened to introduce the respective raw material gases into the mass flow controllers 2211-2216.
After completing the preparation for the film formation as above described, the formation of a give layer on the cylindrical substrate 2112 is conducted, for example, in the following manner.
When the temperature of the cylindrical substrate 2112 becomes stable at a desired temperature, the exit valves 2251-2256 and the sub-valve 2260 are opens as necessary to introduce raw material gases from the gas reservoirs 2221-2226 into the reaction space of the reaction chamber 2111 through the gas feed pipes 2114. Then, the flow rate of each raw material gas is adjusted to a desired valve by the corresponding mass flow controllers 2211-2216. The inner pressure (the gas pressure) of the reaction space of the reaction chamber 2111 is adjusted to a predetermined valve of less than 1 Torr by regulating the opening of the main valve 2118 while observing the vacuum gage 2119. When the inner pressure of the reaction space of the reaction chamber 2111 became stable at the predetermined value, a high frequency power source with an oscillation frequency of 13.56 MHz (not shown) is switched on to apply a high frequency power into the reaction space of the reaction chamber 2111 through the high frequency matching box 2115. Glow discharge is generated to decompose the raw material gases introduced therein whereby causing the formation of a deposited film (for example, a deposited film containing silicon atoms as a matrix) on the cylindrical substrate 2112.
After the deposited film is formed at a desired thickness on the cylindrical substrate, the application of the high frequency power is suspended, and the exit valves are closed to suspend the introduction of the raw material gases into the reaction chamber. The formation of the deposited film is thus completed.
A layer having a multilayered structure can be formed by repeating the above film-forming procedures several times.
In the above film formation, it is a matter of course that all the exit valves other than those required for forming a given layer are closed. Further, after the formation of each layer, if necessary, the inside of the system is evacuated to a high vacuum by closing the exit valves 2251-2256 while opening the sub-valve 2260, and fully opening the main valve 2118 to avoid the gases used for the formation of the previous layer from remaining in the reaction chamber 2111 and in the pipe ways from the exit valves 2251-2256 to the inside of the reaction chamber 2111.
In the case of forming a large area deposited film, for instance, in the production of an electrophotographic photosensitive device (hereinafter referred to as "electrophotographic light receiving member"), in accordance with the foregoing manner, it is necessary for the large area deposited film to have a uniform thickness and a homogeneous film property. In order to achieve this, there are various proposals as will be described below.
Japanese Unexamined Patent Publication No. 213439/1984 discloses a technique in which a cylindrical electrode (which constitutes the circumferential wall of a reaction chamber having a discharge space in which a cylindrical substrate is positioned) is designed to serve also as means for introducing a raw material gas into the discharge space and a plurality of gas ejecting holes are arranged at the wall face of the cylindrical electrode so that the raw material gas can be uniformly introduced into the discharge space, whereby improving the uniformity of the thickness and film property for a deposited film formed on the cylindrical substrate.
Japanese Unexamined Patent Publication No. 44477/1991 discloses a technique where a cylindrical electrode is arranged at a position concentric to a cylindrical substrate arranged in a discharge space of a cylindrical reaction chamber. The cylindrical electrode is designed to serve also as means for introducing a raw material gas into the discharge space and a plurality of gas ejecting holes are arranged at the wall face of the cylindrical electrode at a rate of hole area in the range of 0.1 to 2.0%. A deposited film formed on the cylindrical substrate is prevented from having a pile-like protrusion therein, which will cause a defective image when an electrophotographic light receiving member comprising a deposited film having such pile-like protrusion is subjected to electrophotographic image formation.
Japanese Unexamined Patent Publication No. 30125/1983 discloses a technique where the introduction of a raw material gas into a cylindrical reaction chamber (which serves as an electrode) having a discharge space in which a cylindrical substrate is positioned is conducted by a plurality of gas feed pipes arranged outside the cylindrical substrate in the reaction chamber (specifically, in the discharge space). Each of the gas feed pipes is being provided with a plurality of gas ejecting holes (for introducing the raw material gas toward the cylindrical substrate) such that the cross-sectional area of each gas ejecting hole and the interval between each adjacent gas ejecting holes are varied in the longitudinal direction of the cylindrical substrate so as to uniformly feed the raw material gas into toward the cylindrical substrate in the discharge space. Thus, a deposited film having a uniform thickness on the cylindrical substrate is formed. The deposited film is capable of providing an electrophotographic light receiving member which can prevent an uneven image from occurring in the electrophotographic image formation.
Japanese Unexamined Patent Publication No. 32413/1983 discloses a technique where a cylindrical electrode (comprising the circumferential wall of a cylindrical reaction chamber having a discharge space in which a cylindrical substrate is positioned) serves also as means for introducing a raw material gas into the discharge space or the case where the introduction of a raw material gas into the discharge space is conducted by a plurality of gas feed pipes arranged outside the cylindrical substrate in the reaction chamber (specifically, in the discharge space). A plurality of gas ejecting holes are arranged at cylindrical electrode or the each of the gas feed pipes so that the raw material gas fed can rotate in a predetermined direction, thereby improving the uniformity of the thickness of a deposited film formed on the cylindrical substrate.
Japanese Unexamined Patent Publication No. 479/1988 discloses a technique where the introduction of a raw material gas into a cylindrical reaction chamber having a discharge space in which a cylindrical substrate is positioned is conducted by a gas feed pipe provided with a plurality of gas ejecting holes; and the angles of the gas ejecting holes of the gas feed pipe to the cylindrical substrate, the inner diameter of the cylindrical electrode, and the inner diameter of the cylindrical substrate are specified so that an improvement can be made in the uniformity of the thickness and property of a deposited film formed on the cylindrical substrate without rotating the cylindrical substrate during the film formation.
Japanese Unexamined Patent Publication No. 7373/1988 discloses a technique where the introduction of a raw material gas into a cylindrical reaction chamber having a discharge space in which a cylindrical substrate is positioned by a plurality of gas feed pipes, each being provided with a plurality of gas ejecting holes. The numerical interrelations between the cross section of each of the gas feed pipes and the cross sections of the gas ejecting holes are specified so that an improvement can be made in the uniformity of the thickness and property of a deposited film formed on the cylindrical substrate without rotating the cylindrical substrate during the film formation.
In accordance with these techniques, it is possible to produce an electrophotographic light receiving member having a light receiving layer comprising a deposited film with an improvement in the uniformity of the thickness and film property at a reasonable yield.
However, the electrophotographic light receiving member still has several points required to be further improved in order for it to have further improved overall characteristics.
Particularly, in recent years, electrophotographic apparatus in which an electrophotographic light receiving member is used have been rapidly progressed to have a high driving speed, an improved image-reproducing performance capable of providing a high quality image at a high speed, and a prolonged durability. Specifically, an improvement has been made in the optical exposure instrument, development instrument and transfer instrument used in the electrophotographic apparatus.
In addition, in recent years, there is a societal demand for further improving the quality of an image reproduced by the electrophotographic apparatus.
In view of this, for the electrophotographic light receiving member, there is an increased demand for further improving it so as to be compatible with such advanced electrophotographic apparatus and to satisfy the above societal demand. As a consequence, the electrophotographic light receiving member, is required to further improve its electric and photoconductive properties and to further progress such that it exhibits a further improved image-forming performance under any environmental use conditions while satisfactorily maintaining charge retentivity and photosensitivity.
Under this circumstance, in accordance with the foregoing techniques, it is possible to attain an electrophotographic light receiving member having a light receiving layer satisfactory in terms of the uniformity in the thickness and property to a certain extent and which can meet the above requirements to a certain extent. However, in view of further improving the quality of an image reproduced, the electrophotographic light receiving member is not always satisfactory. Particularly, for an amorphous silicon series electrophotographic light receiving member, there still is a demand for forming a high quality amorphous silicon deposited film having more uniform thickness and more uniform property as its light receiving layer and which effectively prevents the occurrence of a minute defects in reproduced image. In order to achieve this demand, it is necessary to at least make the flow rate of a raw material gas and the film deposition rate in the film-forming chamber upon the formation of the amorphous silicon deposited film uniform. Besides, there is an occasion such that foreign matter such as polysilane, which is deposited on the inner wall face of the film-forming chamber during the film formation, is removed to fly on a substrate on which a film is being deposited whereby contaminating the film, where the film is abnormally grown. When the film thus abnormally grown is used as a light receiving layer of an electrophotographic light receiving member, the electrophotographic light receiving member has a tendency to cause a minute defect in a reproduced image. Therefore, it is necessary to prevent such foreign matter, i.e., polysilane, from flying to the substrate during the film formulation.